Liquid development of electrostatic latent image

ABSTRACT

Electrophotographically produced electrostatic latent images are developed with minimal toner coloration of background or highlights. Residual electrostatic charge or potential is neutralized before development by a uniform application to the latent image surface of particles carrying a charge opposite in polarity from that of the electrostatic image. The quantity and population density of these charged particles are selected to just neutralize said residual potential. These particles are either colorless or have a coloration that is non-contrasting with the image background. After neutralizing the residual potential, development of the latent image is effected by colored toner particles, which will deposit in the image charge areas only.

United States Patent [191 Sato et a1. 1

[451 Apr. 30, 1974 LIQUID DEVELOPMENT OF ELECTROSTATIC LATENT IMAGE [73]Assignees Xerox Corporation, Stamford, Conn.

[22] Filed: Mar. 13, 1972 [21] Appl. No.: 234,192

[52] US. Cl. 117/37 LE, 96/1 LY, 355/10 51 Int. Cl C03g 13/10 [58] Fieldof Search 117/37 LE; 96/1 LY; 355/10; ll8/D1G. 23, 637

[56] References Cited UNITED STATES PATENTS 3,527,684 9/1970 York et a1117/37 LE 3,081,263 3/1963 Metcalfe et al. 96/1 LY 3,212,888 10/1965Weugebauer 117/37 LE 3,010,842 11/1961 Richer 117/37 LE 3,300,410 H1967Oliphant 1 17/37 LE 2,991,754 7/1961 Johnson 355/10 Metcalfe et a1 96/1.5

3,212,887 lO/1965 Miller et a1. 96/1.2 3,236,776 2/1966 Tomanek 96/1 LY3,262,806 Gource l17/l7.5

Primary ExaminerWilliam D. Martin Assistant Examiner-M. Sofocleous [57]ABSTRACT Electrophotographically produced electrostatic latent imagesare developed with minimal toner coloration of background or highlights.Residual electrostatic charge or potential is neutralized beforedevelopment by a uniform application to the latent image surface ofparticles carrying a charge opposite in polarity from that of theelectrostatic image. The quantity and population density of thesecharged particles are selected to just neutralize said residualpotential. These particles are either colorless or have a colorationthat is non-contrasting with the image background. After neutralizingthe residual potential, development of the latent image is effected bycolored toner particles, which will deposit in the image charge areasonly.

7 Claims, 4 Drawing Figures LIQUID DEVELOPMENT OF' ELECTROSTATIC LATENTIMAGE INTRODUCTION AND BRIEF SUMMARY OF INVENTION The present inventionrelates to the development of electrostatic latent images, and moreparticularly it is concerned with the elimination or minimizing of tonerdeposition onimage background or highlight areas.

In electrophotography it would be desirable for the charge onelectrophotographic surfaces to be discharged completely in the imagehighlight or background areas. In practice however, a residual charge orpotential is usually present in these areas, and on development of theelectrostatic latent image, this residual charge causes undesirabletoner deposition, resulting in background coloration and dullhighlights. In one .known method of preventing toner deposition due toresidual potential, a bias potential is applied from an external sourceto the conductive backing of the electrophotographic coating duringdevelopment, tocompensate or neutralize the residual potential. The biaspotential approach complicates the developing apparatus, and it hasafurther disadvantage in that toner deposits on the counter electrode,and this tends to smudge the image surface of the electrophotographicmaterial being developed.

In accordance with the present invention the foregoing disadvantages arelargely overcome, and developed prints are produced with brighthighlights and minimal background toner coloration. These results areattained by applying uniformly over the electrostatic latent imagebearing surface of the electrophotographic material, a dispersion ofcolorless charged particles and causing the particles to uniformlyadhere to the image surface area. These charged particles have a chargeof opposite polarity from that of the latent image, and are deposited ina quantity and population density selected substantially to justneutralize the residual potential. This first deposition is followed bya .usual development process employing conventional colored tonerparticles. Since the colorless particle deposit substantiallyneutralizes the residualpote'ntial over the image surface, deposition ofcolored toner on the highlight and background areas during thedevelopmentprocess is essentially eliminated. It is stated that thecharged particles are colorless. This condition is preferred, butobviously the particles could have a coloration that is not contrastingwith the background coloration. For example, if the photoelectric imagesurface is white, these particles could be white, if desired.

It is apparent that deposition of the colorless particles must beuniform over the entire image area, and it must also be controlled sothat it effects only aneutralization of the residual potential. Caremust be taken to avoid preferential deposition of this material indevelopment quantities over theimage charge areas, for such depositionwould diminish the density of the image attainable in the developmentstep. Accordingly, pursuant to the present invention a suspension of theneutralizing particles in a liquid vehicle is applied as a very thinuniform film over the entire latent image bearing surface, and

at the same time, a development electrode is placedas close as possibleand in parallel relation to said surface so as not to allow theparticles to deposit preferentially onto the areas of high chargedensity. The presence of maximum density over background obtained bysubsequent development with colored toner.

It is accordingly one object of thepresent invention to provide forreduced'background deposition of toner and brighter highlights'in thedevelopment of electrostatic'latentimages on electrophotographicsurfaces.

Another object of thepresent invention is to provide such results byneutralizing the residual, potential on the electrophotographicsurfaces.

And still another object of the present invention is to effect suchneutralization by the uniform deposition overjthe latent image surfaceof charge neutralizing particles that do not affect'the color renditionof the developertoner particles and do nothave a colorationthat'contrasts with that of the background.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art.

from a consideration of the following detailed description of exemplaryembodiments of the invention, had

in conjunction with the accompanying drawings in which like numeralsrefer to like or corresponding parts, and wherein:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a three part drawing depictingan image charge pattern on an electrophotographic surface in part A, theapplication thereto of residual potential neutralizing particles in partB, and the application of developer toner thereto in part C and FIG. 2is a schematic representation of apparatus for practicing the presentinvention.

DETAILED DESCRIPTION Referring to FIG. 1, numeral 10 designates aconventional sheet of electrophotographic paper having a photoconductivesurface on one side. As is well understood in the art, an electrostaticlatent image is formed thereon by first applying a uniform electrostaticcharge to the photoconductive surface, and then exposing the surface toan optical image while the back of the sheet 10 is grounded..Asa result,the electrostatic charge is discharged in the illuminated areas andretained in the non-illuminated areas, providing an electrostatic latentimage.

A negative charge electrostatic latent image is depicted in FIG. 1A.Portion ll depicts an area that was illuminated during the exposurestep, and the charge is removed therefrom except for a small residualcharge or potential. During development, a small amount of toner wouldadhere to portion 12, thereby toning the background, if area 12 isbackground, or dulling the highlight, if area 12 is a highlight.

In accordance with the present invention, this residual potential isneutralized by applying a small quantity of positively charged particlesuniformly over the photocond'uctive surface of sheet 10. This result isdepicted in FIG. 18, where particles 13 have been uniformly applied overthe entire latent image. The concentration of particles 13 isselected-so as just to neutralize the residual charge in area 12 ofsheet 10, as shown in FIG. 1B. A similar concentration ofchargedparticles 13 is also present in area 11, but since the electrostaticcharge in area 11 is so much higher than in area 12, area-11 remainsunneutralized. The unneutralized charge in area 11 corresponds to thefull image charge, since only the non-image residual charge portion hasbeen neutralized.

In order not to interfere with the subsequently developed image, andindeed to improve it as is the purpose of this invention, the chargedparticles 13 are selected to be either colorless or of the same color asthe surface of paper 10. r

After the residual charge is neutralized by application of particles 13,the sheet is developed in a conventional way, and colored tonerparticles 14 are caused to adhere to the surface of sheet 10 in thecharged area 11, as depicted in FIG. 1C. No toner particles adhere inthe area 12, because all residual charge has been fully neutralized byparticles 13. Thus, the background or highlight area 12 remains clear.

FIG. 2 schematically shows an apparatus for practicing the processdescribed above and illustrated in FIG. 1. Tank contains a dispersion orsuspension 21 of positively charged colorless particles 13 in anelectrically insulating liquid vehicle. A series of rolls 22 arepartially immersed in the dispersion 21 and are paired with companionpressure rolls 23. Photoconductive sheet 10 is caused to enter the nipsof the paired rollers 22 and 23 and to pass therethrough with itselectrostatic latent image surface faced downward toward rolls 22, rolls22 and 23 rotating in the directions indicated by the associated arrows.As rolls 22 rotate, each carries a film of dispension 21 on its surfaceto the image surface of sheet 10, causing a controlled and uniformdeposition of particles 13 on the surface of sheet 10. Rolls 22 arepreferably polished stainless steel and function as developmentelectrodes. By this operation, and by control of the concentration ofparticles 13 in the dispersion 21 and control of the thickness of thedispersion film carried on the rolls 22, the residual charge orpotential on the image surface of electrophotographic sheet' l0 issubstantially exactly neutralized by the particles 13, deposited onsheet 10, as depicted in FIG. 1B.' 7

After the sheet 10 passes the rolls Hand 23, it follows a feed pathindicated by the dashed line 24 and the arrow 28. Any excess dispersion21 carried by sheet 10 v is removed by squeeze rolls 25. The sheet 10 isthen directed and fed by rolls 26 into developer tank 29 containing aconventional developer suspension 30 of colored toner particles 14 in anelectrically insulting liquid In the foregoing operations, in order toobtain a completely uniform application of charged particles 13 to thesurface of sheet 10, it would be necessary to have an infinitely smallgap between rolls 22 and the surface of sheet 10. However, as apractical matter a finite gap is always necessary in order to accomodatethe presence of a film of dispersion 21. Therefore, it is impossible toeliminate completely the tendency of the toner particles 13 to adheresomewhat preferentially to the areas of higher or image charge density.From a practical point of view, however, such preferential deposition isnegligible, since the amount of toner deposition in the actualdevelopment step remains quitedominant. In order to realize a moreperfectly uniform deposition of charged particles 13, one may resort toa method in which a bias potential far larger than the maximum potentialin the latent image is applied to the development electrode during thecharged particle deposition. It will of course be appreciated by thoseskilled' in the art that before sheet 10 is treated by despersion-21, itmay be prewetted with a suitable electrically non-conducting liquid freeof any particles or toner, to prevent mechanical absorption of thedispersion 21 into the porous paper sheet 10.

To insure proper operation of the present method, it is of coursenecessary that the charge forming the latent image should decay at onlya very slow rate during the processing steps. Since the first coatingstep does not contribute to the optical density of the finally developedimage,'but rather to the reduction of highlight or backgroundcoloration, dissipation of charge in this phase of the process must beminimized. The rate of charge decay on the surface of sheet 10 dependslargely on the composition of the photoconductive insulating coating andon the treating liquid with which the surface is treated. The decay rateis smaller in the more insulating treating liquids, and in those whichhave a lesser tendency to dissolve or swell the resinous ingredientsincluded in the photoconductive coating. A variety of resinous materialsare suitable for the photoconductive coating, when during thisdevelopment process the coating is wetted with insulating liquids havinglow dissolving power, such as halogenated and isoparaffinic hydrocarbonsolvents. When using insulating liquids such as Decalin, Tetralin,kerosene, or'cyclohexane, which have a higher dissolving power, resinousphotoconductor compositions having polar structures are desired.Suitable examples include alkyd, epoxyester, or linear polyester resinscross-linked by suitable reagents, such as polyisocyanate compounds.With photoconductive insulating coatings having these bindercompositions, surface charges decay very slowlyin the insulatingprocessing liquids.

The dispersion 21 used to effect the uniform coating of chargedparticles can be prepared according to the methods known for thepreparation of conventional liquid electrophoretic developers used inelectrophotography. Suitable white pigments that may be used for theparticles 13 in this dispersion include titanium oxide,

zinc oxide and, as the most desirable, various body pigments such ascalcium carbonate, alumium hydroxide, barium sulfate, alumium'oxide',talc, silica, calcium silicate, magnesium carbonate, magnesium oxide,etc. While no elaborate charge control techniques are necessary so longas the pigment particles have a distinct tendency to be charged in adefinite polarity in insulating liquids, any of the known liquiddeveloper charge control techniques may be employed, such as theincorporation of resinous control agent in cases where theelectrophore'tic property of the pigment particles are unsuitable.Colorless particles 13 may be formed from clear resin particles orlow-molecular weight resin powders. The desirable range of particle sizeis less than several microns in diameter.

' To further illustrate the practice of the present invention thefollowing specific examples are presented:

. EXAMPLE I coating comprising photoconductive zinc oxide and alkydresin binder cured by a polyisocyanate compound, was charged to -l00volts, and exposed .to a continuous tone image to form an electrostaticlatent image. Both of the above-described suspensions includedpositively charged particles or toner. The latent image bearing sheetwas then processed in the manner.

EXAMPLE II Calcium carbonate was used in place of the aluminum hydroxidefor the coating dispersion 21 described in Example I. .A commerciallyavailable black devel- 'oper for Electrofax copier was used as theliquid developer. The electrostatic latent image was formed on a zincoxide/resin binder type photoconductive sheet, using an alkyd resinbinder cured with cobalt naphthenate. When the electrostatic latentimage was treated and developed in accordance with the proceduresdescribed above, an excellent image was obtained, free of backgroundcoloration.

EXAMPLE lll A methylethyl ketone solution of vinyl chloride/vinylacetate copolymer (copolymer composition VCliVAc=70:30, degree ofpolymerization-260) was poured into kerosene containing linseed oilunder agitation by ultrasonic energy. The resulting uniform despersionwas used as the coating dispersion 21. For developer 30, a dispersioncomprising the following ingredients was first formed:

Microlith Blue 4G-K (Ciba Ltd.) 203.

Acetone 500ml. Toluol 300ml. Methylethyl Ketone 200ml.

(Microlith Blue 4G-K is composed of a blue chloride/vinyl acetatecopolymer 85:15). 100 ml. of this dispersion was dispersed by means ofan ultrasonic blender into the following mixed solvent:

pigment and vinyl Kerosene 2500ml. Cyclohexane 7000ml. Linseed oil500ml.

The particles in both dispersions were charged negatively. Anelectrostatic latent image of positive polarity was formed on an organicphotoconductive coating mainly comprising polyvinylcarbazol, and it wasprocessed in the manner described hereinabove. An excellent image freeof background coloration was obtained.

What is claimed' is: 1. A method of developing an electrostatic latentimage formed on an electrophotographic surface comprising applying overthe image surface a substantially uniform coating of particles having acoloration which does not contrast with the image background coloration,said particles carrying a charge of opposite polarity from that of theimage charge, the density of said particles in said coating beingselected substantially just to neutralize the residual potential on saidsurface in background and image highlight areas, and thereafterdeveloping said latent image with a developer having colored tonerparticles.

2. A method as set forth in claim 1, wherein the step of applying saidcoating particles to said image surface includes usinga thin film of adispersion of said particles in an electrically insulating liquidvehicle.

3. A method as set forth in claim 2, wherein said coating particles arecolorless.

.4. A method as set forth in claim 3, wherein said developer is adispersion of said toner particles in an electrically insulating liquidvehicle.

5. A method as set forth in claim 1, wherein said developer is adispersion of said toner particles in an electrically insulating liquidvehicle.

6. A method as set forth in claim 5, wherein said coating particles .arecolorless.

7. A method as set forth in claim 5, wherein the step of applying saidcoating particles to said image surface includes using a thin film of adispersion of 'said particles in an electrically insulating liquidvehicle.

2. A method as set forth in claim 1, wherein the step of applying saidcoating particles to said image surface includes using a thin film of adispersion of said particles in an electrically insulating liquidvehicle.
 3. A method as set forth in claim 2, wherein said coatingparticles are colorless.
 4. A method as set forth in claim 3, whereinsaid developer is a dispersion of said toner particles in anelectrically insulating liquid vehicLe.
 5. A method as set forth inclaim 1, wherein said developer is a dispersion of said toner particlesin an electrically insulating liquid vehicle.
 6. A method as set forthin claim 5, wherein said coating particles are colorless.
 7. A method asset forth in claim 5, wherein the step of applying said coatingparticles to said image surface includes using a thin film of adispersion of said particles in an electrically insulating liquidvehicle.